A by-election victory could secure a TV debate place for Farage

The defection of one MP or another towards the end of a parliament is nothing particularly unusual.Â The decision of one to resign and re-contest his or her seat is.Â Were it not for the vote of even greater significance taking place in Scotland next month, the Clacton by-election could have been the seminal political moment of the parliament.Â Depending on the two results, it still might be.

A Scottish Yes would have such profound implications it deserves a thread of its own (before the vote), to game the likely effects on the parties, their leaders and the reaction in the country at large, to work out where any betting value may lie.Â The effect of a No would be less significant though the last time the SNP failed in a referendum, they parliamentary party took a hammering at the next election.Â Itâ€™s also perhaps worth noting that Alistair Darling is 11/1 with Paddy Power to be the next Chancellor and 20/1 to be next Labour leader.

If we assume a No for these purposes, then the attention of the media and of Westminster will rapidly move on to the Essex coastal constituency of Clacton.Â Itâ€™s a measure of how rapidly UK politics is changing that the best odds available on UKIP at the time of writing were 1/4.Â Only twice since WWII has a party other than the Tories, Labour or Lib Dems (or their predecessors) won an English by-election: George Galloway was one, earlier this parliament; Dick Taverne the other, who held his seat at the 1973 Lincoln by-election after his parting of the ways with Labour.

Those odds donâ€™t look tempting to me.Â UKIP has not made the most sure-footed of starts to their campaign with their previously selected candidate refusing to stand down.Â While itâ€™s right that theyâ€™re odds-on favourites at the moment, it wouldnâ€™t take much to turn a spat into a shambles if they canâ€™t sort their local aspect quickly.Â Indeed, much will turn on local matters: how seriously Labour tries (both for their own sake and the indirect effect their campaign, or non-campaign, has on the Blue and Purple camps), how many of Carswellâ€™s activists follow him across, how many UKIP activists are willing to campaign for their erstwhile opponent â€“ and so on.

Even so, Clacton has been described by the most favourable seat for UKIP in the country,Â an assertion thatÂ the European election results reinforce.Â

Thatâ€™s a huge advantage in this election but itâ€™s also a huge risk: if they donâ€™t win, it undermines any claim they have to be taken seriously next year, it would halt their current momentum and would put a hefty spring in the stride of their conquerors.Â On the other hand, if they do make history and gain their first elected MP, then that removes another obstacle to Farage appearing in the leadersâ€™ debates next year â€“ an aspiration that should be UKIPâ€™s number one campaign objective given UKIPâ€™s still-developing activist base, the impact the debates had in 2010 and Farage’s distinctiveness.Â For that reason alone, the odds of debates happening at all should lengthen if UKIP wins.

There is another angle to consider, that of electoral fairness.Â A UKIP win, consolidating their position as the fourth major national party, would go still further to undermining the legitimacy of FPTP; a system that only really works well with two dominant parties.Â I wrote in March that the Toriesâ€™ 2015 manifesto should include a commitment to introducing PR (open lists in constituencies of about five MPs would be best).Â The events this week have made that all the more necessary.